This invention relates generally to cable television receiver systems and particularly to cable systems that receive both scrambled and unscrambled television signals.
Recently the EIA (Electronics Industries Association) has proposed a color interface standard IS105 which defines certain requirements for cable compatible television receivers. Specifically, the proposed standard requires that a cable decoder, which is called a set back box, be interfaced in a particular way with the television receiver for performing prescribed tasks. The interface is a combination of IF/AGC, digital control and baseband audio/video lines required to interconnect cable-ready devices and cable decoders. The decoder is external to the television receiver (or cable-ready device) and is connected to it via the interface. The decoder may include a descrambler that enables the user to recover scrambled television signals or other encrypted services. Both the television receiver and the decoder include microprocessors for controlling their various respective operations. The decoder interface is also connected to the receiver via a control line to enable the two microprocessors to "talk to" each other.
A very common form or video scrambling involves suppression of synchronizing signals (sync) and random inversion of video. Generally the audio signal, which is FM modulated, is not affected. In most television receivers difficulties arise because the television receiver, when receiving a scrambled video signal, is unable to restore the sync to its proper level which is the basis for developing the AGC signal. During scrambled signal reception, the video signal is provided by the set back decoder. Switching means are provided for delivering the video from the receiver or decoder as appropriate. The audio generally is delivered from the receiver in all cases.
A difficulty arises in developing the audio signal in the television receiver when a scrambled video signal is received. This is because the television receiver AGC system cannot effectively control the signal level when the signal is scrambled. Even though the audio is an FM modulation signal, certain signal conditions create audio dropouts and very unusual noises which can be very annoying. The present invention provides a relatively simple, cost effective method of properly developing the audio signal in the television receiver under conditions where the decoder is supplying the video signal.